A number of genetic systems of immunologic significance are being studied in rabbit populations which are undergoing progressive inbreeding. Part of the work consists of identifying blood group systems and the mechanism for the appearance of antigens resulting from allelic interactions. The linkage relationships of 6 blood group loci, several serum protein polymorphism and the major histocompatibility locus (RL-A) with other known linkage groups in the rabbit are under investigation and the selective significance of these genes is being evaluated. New histocompatibility loci are being sought and assay systems for these loci are being developed by the use of cell culture methods. Attempts are being made to determine the role of serologically detected polymorphism in natural selection occurring during intensive inbreeding. Several loci under examination show higher frequencies of heterozygosis than would be expected on theoretical grounds. The induction of tolerance and graft vs. host disease in RL-A characterized newborn rabbits is under investigation.